1. Technical Field
The technical field relates to a wireless power transfer system configured to wirelessly transfer electric power in a noncontact manner by utilizing resonant magnetic field coupling, a wireless power transmitter apparatus for use in the wireless power transfer system, and a frequency controller apparatus and a power transfer method to be used for the wireless power transmitter apparatus.
2. Description of the Related Art
A Patent Document 1 discloses a wireless power transfer apparatus to transmit energy via a space between two resonant circuits. In this wireless power transfer apparatus, vibrational energy is wirelessly transmitted (in a noncontact manner) by coupling of two resonant circuits via leaching out (evanescent tail) of vibrational energy at a resonant frequency occurring in a peripheral space of the resonant circuits.
Patent documents related to the present disclosures are as follows:
Patent Document 1: Specification of USP application publication No. 2008/0278264 (FIGS. 12 and 14)
Patent Document 2: Japanese patent publication No. 4314709
Patent Document 3: Japanese patent publication No. 4281362
In order to improve long-term reliability of an electronic apparatus such as a wireless power transfer apparatus, it is necessary to reduce electrical stress to the elements that configure the electronic apparatus. The switching power apparatus described in the Patent Document 2 changes the oscillation frequency of a switching signal to drive the switching elements from a high frequency to a low frequency at the time of starting up the switching power apparatus, reducing damages to the switching elements. Moreover, the discharge lamp lighting device described in the Patent Document 3 reduces in steps the frequency of square waves applied from the DC-AC converter circuit to the resonant circuit in the startup interval.
Generally speaking, in a wireless power transfer system, the transmission frequency is adaptively controlled during the power transfer in order to maintain satisfactory transmission efficiency even when the power and current supplied to the load, the inputted voltage and current or a coupling coefficient between the power transmitting antenna and the power receiving antenna fluctuates or when the characteristics of the power transmitting antenna and the power receiving antenna differ from the designed values attributed to manufacturing variations and the like. However, the Patent Documents 2 and 3 describe no method of reducing the electrical stress to the components or elements of the system during the operation of the wireless power transfer system. Moreover, it is difficult to control changing of the transmission frequency during the power transfer so as not to give, for example, transitional electrical stress to the components or elements in the wireless power transfer circuit described in the Patent Document 1.
One non-limiting and exemplary embodiment provides a wireless power transfer system capable of relieving electrical stress to the components or elements as compared with the prior art, a wireless power transmitter apparatus for use in the wireless power transfer system, and a frequency controller apparatus and a power transfer method for use in the wireless power transmitter apparatus.